The present invention relates generally to continuously operating sedimentation tanks wherein the settled solids or sludge are continuously removed from the tank bottom with a rotary rake structure that conveys the sludge or settled solids to a sump or center discharge opening. The operation of such units is ordinarily continuous.
In large tanks, which may exceed 100 feet in diameter, the raking structure that is required to move the sludge to a center discharge opening is generally very massive to overcome the resistance of the sludge. In some instances, the sludge may become excessively heavy which may result in overloading the power source which drives the rake structure. In other instances, foreign substances such as rocks may be accidentally introduced into the tank and result in an obstruction to the raking movement of the rake arm.
To overcome this problem, it has been proposed to pivotally support the rake structure on the rotatable driven member so that the rake structure can be pivoted upwardly when the rake arm encounters heavy resistance to movement. Such an arrangement is generally disclosed in Scott U.S. Pat. No. 2,122,384. The apparatus disclosed in the Scott patent is constructed so that the rake arms can pivot upwardly when an obstruction is encountered.
More recently various alternate proposals have been developed for accomodating movement of the rake arm when an excessive load is encountered by the motor or when an obstruction is located in the tank. Examples of such devices are disclosed in U.S. Pat. Nos. RE.27,000; 3,542,207; and 3,833,126. In all of these patents, the rake structure is pivoted in some respect on a member that is rotated about the center of the tank and cables extend from the rake structure and are attached to fixed booms that extend from the rotating member adjacent the upper end thereof. All of these mechanisms provide for an automatic lifting of the scraper arms at some specified torque level to prevent an excessively high torque load from being developed in the drive motor and also allow the use of smaller motors.
In these devices, the boom or drag arm leads the rake arm so that the cables are at an angle with respect to the bottom of the tank. Thus, an upward force is developed in the cables as a function of the horizontal tension force due to the drag resistance of the sludge on the rake arms. When the vertical force exceeds the dead weight load of the rake arm, the rake arm will swing upwardly about the pivot at the inner end thereof.
In devices of this type, one of the shortcomings is the fact that the torque must continually increase in order for the rake arm to continue to raise to a new balanced position. Another problem encountered with devices of the above type is the fact that the torque force at which the rake arm will pivot upwardly is not readily adjustable for different conditions. It will be appreciated that the primary factor in determining at what torque force the rake arm will pivot is the dead weight or total weight of the rake arm which, of necessity, is dependent upon the structural requirements of the unit.